PowerPC :: G4 RAM Modules And Error Code

I have an old PowerMac G4 (AGP Graphics), which has served me dutifully for a long time. However, I recently took all but 128mb of the ram out to temporarily use in other computers, and when I went to put the modules back in, basically none of them worked. Here's the deal: I have 2 256 modules, and when I put those in my computer, whether individually, together, or with other modules, I get this message on-screen and the computer does not boot: localhost:/ root# []. Similarly, I have 3 128mb sticks, but if I put more than one in the computer at a time, I get the same message. I'm thinking the computer may have some issue with more than 128mb of ram, but why all of a sudden? I don't really know anything about code or whatever the computer is trying to tell me.